No. 594] 



COLOR MUTATIONS IN MICE 



343 



lowish tinge to the under surface. The dorsal appear- 

 ance of this animal was apparently the same as the other 

 agoutis of this generation, a deep rich agouti with much 

 black pigment. The ventral surface was, however, dis- 

 tinctly lighter in color than the other agoutis and showed 

 a decrease in dark pigment as compared to them. 



The yellow-bellied variation reappeared in the imme- 

 diate progeny of the original yellow-bellied female and 

 a race of these animals was established. When single 

 mating tests were made to determine its behavior it was 

 found that when yelloiv-bellied agoutis one of tvhose par- 

 ents was a non-agouti were crossed with non-agouti, only 

 yelloiv-bellied agouti and non-agouti young resulted. 

 This fact indicated that yellow-bellied agouti fell some- 

 where in the series of multiple allelomorphs recorded by 

 Cuenot and by Morgan. 



At about this time English "black and tan" mice 3 

 crossed with white-bellied agoutis were found to give 

 among their progeny agouti young almost identical with 

 the yellow-bellied agoutis described above. 



It at once suggested itself that the yellow-bellied 

 agoutis occurring in my selection experiment were really 

 white-bellied agoutis with one or more modifying factors 

 which encouraged a higher degree of pigmentation than 

 is normally found. This increase in an oxidation process 

 would account for yellow pigment appearing in the tips 

 of the ventral hairs which in ordinary white-bellied 

 agoutis are unpigmented. 



This supposition was further favored by the fact that 

 as certain of the yellow-bellied agoutis grew to be old 

 mice they showed a diminishing depth of pigmentation, 

 and developed typical white-bellied agouti coat color. In 

 old age they were not visibly different from some of the 

 darkest white-bellied agoutis descended from a male of 

 this variety kindly sent me by Professor Morgan. 



3 Black and tan mice are a very dark type of yellow. Only a small amount 

 of yellow appears on the sides, while the ventral surface is a deep rich 

 reddish yellow. Black animals descended from black and tans are coal 

 black with deep black ears, feet and tail. 



